Horizontal sync detection circuits described in the prior art literature are generally intended for home TV or other commercial video applications. These prior art circuits perform satisfactorily in applications which have very limited temperature variations. When the aforementioned prior art circuits are used in military applications, which can vary from -55.degree. C. to +125.degree. C., extensive modifications are required to the transistors, diodes, resistors and capacitors in order to achieve temperature compensation. The prior art horizontal sync circuits do not allow for a two-to-one or greater variation in video signal amplitude, particularly where the video signal being processed is taken from a video tape recorder. Existing detector circuits typically clamp the negative peaks of the horizontal sync pulses to a fixed level and have a fixed threshold above the clamping level to sense the presence of a horizontal sync pulse in the video data stream. The problem with such prior art circuits is that the clapming levels and the threshold levels are generally established by diode drops or transistor base-emitter drops which are not temperature compensated. See FIG. 1 for a composite video wave form. It may be observed from the waveform that, for example, if the sync detector triggering threshold is set for a fixed 125 mV above the negative peak, the mid-point of the horizontal sync pulse, the ideal point for a nominal level signal, trouble develops if the signal is reduced to less than one-half the nominal value. The problem being that with triggering threshold fixed, the signal moves up into the video blanking levels. Thus it can be seen that the triggering point must be adjustable so that it is proportional to the composite video signal level.
The negative peaks of the horizontal pulses, the points of concern, vary in amplitude for several reasons. Different sources have various levels even though standards, such as EIA Standard RS-170 and RS-232. However, even if the standards were adhered to, degradation can occur and in most cases, the input video is capacitively coupled so that the peak amplitude of the horizontal pulses is dependent on the video content of the signal. That is, the negative peaks move up or down depending on the picture content, the average value is zero for a capacitive coupled signal. Thus a means for sensing the variations of the peaks of the horizontal signal is needed in all cases.